JP6708305B2 - Silicon wafer polishing method - Google Patents

Description

The present invention relates to a silicon wafer polishing method characterized by a final polishing step of mirror-polishing one surface of a silicon wafer that has been double-sided polished.

The process for producing a silicon wafer mainly includes a single crystal pulling step for producing a single crystal ingot and a processing step for the produced single crystal ingot. This processing step generally includes a slicing step, a lapping step, a chamfering step, an etching step, a mirror polishing step, a cleaning step and the like, and a silicon wafer having a mirror-finished surface is manufactured by going through these steps.

In the mirror-polishing step, multi-step polishing such as a double-sided polishing step (rough polishing step) of simultaneously polishing both surfaces of a silicon wafer and a final polishing step of mirror-finishing one side of the silicon wafer is performed. Generally, the final polishing step is performed by using a polishing unit including a surface plate having a polishing pad on the surface thereof and a head for holding a silicon wafer. One side of the silicon wafer held by the head is pressed against the polishing pad, and while the polishing liquid (polishing slurry), which is an alkaline aqueous solution containing abrasive grains, is supplied to the polishing pad, both the head and the surface plate are rotated. As a result, one surface of the silicon wafer is polished by mechanochemical polishing (CMP) in which the mechanical polishing action by the abrasive grains and the chemical polishing action by the alkaline aqueous solution are combined, and becomes a mirror surface having excellent smoothness.

Here, in the final polishing step, two or more stages of polishing including one or more pre-stage polishing steps performed in one or more pre-stage polishing units and a finish polishing step performed in the finish polishing unit thereafter are performed. In Patent Document 1, in the final polishing step of the final polishing step, an alkaline aqueous solution containing a water-soluble polymer and abrasive grains having a density of 5×10 ¹³ particles/cm ³ or less is used as a polishing liquid to obtain PID( It is described that a silicon wafer with reduced Process Induced Defect) can be obtained.

Further, in Patent Document 2, a first polishing step of polishing one surface of a silicon wafer while supplying a polishing solution containing a water-soluble polymer-free alkaline solution containing abrasive grains as a main component, , Following the first polishing step, by supplying a protective film forming solution containing a water-soluble polymer to the polishing cloth after the first polishing step, in the first polishing step of the silicon wafer A protective film forming step of forming a protective film on the surface to be polished by bringing the protective film forming solution into contact with the surface to be polished, and an alkaline aqueous solution containing a water-soluble polymer and containing abrasive grains as a main component While supplying a polishing liquid to a polishing cloth different from the polishing cloth used in the first polishing step, a second polishing step of polishing the protective film formation surface of the silicon wafer formed in the protective film formation step, A method for polishing a silicon wafer, which is characterized in that it is provided, is described. This polishing method is a technique for reducing the occurrence of a watermark, which is a problem when the silicon wafer is transported in the air and transferred from the first polishing step to the second polishing step, by forming the protective film.

International Publication No. 2010/140671 JP, 2016-51763, A

The PID evaluated in Patent Document 1 is a linear protrusion defect as shown in FIG. 1(a) of Patent Document 1, and the generation mechanism thereof is considered as follows. That is, during the final polishing process, the abrasive grains and other foreign matter in the polishing liquid scratch the surface of the silicon wafer with a certain probability. Since this linear scratched portion becomes a work-affected layer, the etching rate becomes lower than that of other portions on the wafer surface in the subsequent etching steps such as the cleaning step immediately after finish polishing and the final cleaning step after the inspection. The result is a linear protrusion. Then, in Patent Document 1, the wafer surface is measured with a laser particle counter (KLA Tencor, SP2), and a defect having a size of 35 nm or more and classified into LPD-N is identified as PID, and the number thereof is evaluated. There is.

According to the wafer polishing method of Patent Document 1, it is possible to surely suppress the generation of PID by setting the number of abrasive grains to 5×10 ¹³ particles/cm ³ or less in the final polishing step. However, as a result of examination by the present inventors, it was found that the wafer polishing method of Patent Document 1 could not sufficiently suppress the occurrence of scratches having a relatively small step. This scratch with a small step is revealed by measuring the wafer surface in the normal mode of a laser particle counter (KLA Tencor, SP3), and is a defect different from the PID evaluated in Patent Document 1. is there.

In addition, the wafer polishing method of Patent Document 2 does not consider reducing scratches having a small step as described above. According to the study of the present inventors, the occurrence of scratches having a relatively small step is generated. It was found that it was not possible to sufficiently suppress.

Therefore, in view of the above problems, it is an object of the present invention to provide not only a PID but also a method for polishing a silicon wafer capable of suppressing the occurrence of scratches having a small step.

The present inventors have conducted extensive studies to solve the above problems, and have found the following findings. In the conventional final polishing step, the pre-stage polishing step uses a polishing solution with a relatively high alkali etching rate and a large number of abrasive grains to perform polishing with a strong polishing action. And a polishing liquid having a small number of abrasive grains was used to perform polishing to achieve higher flatness. The wafer immediately after the first-stage polishing process is transferred to the finish polishing unit while being sprinkled with water. At this time, due to the above characteristics of the polishing liquid used in the former polishing step, the wafer surface immediately after the former polishing step is a water repellent surface. The present inventors considered that if the wafer was transported while the water surface was a water-repellent surface and the final polishing was performed, agglomeration of abrasive grains might occur on the wafer surface at the stage of the final polishing. Therefore, in the middle of the pre-polishing process, the polishing liquid supplied to the wafer surface is changed from the pre-polishing polishing liquid to the polishing liquid for finish polishing, and the wafer surface immediately after the pre-polishing process is hydrophilized to finish. The idea was to prevent the agglomeration of abrasive grains on the wafer surface at the polishing stage. Then, when the pre-stage polishing process and the final polishing process were performed in this manner, it was confirmed that scratches with a low level difference on the wafer surface were reduced.

The present invention has been completed based on the above findings, and its gist configuration is as follows.
(1) Using a first polishing plate having a first polishing pad on its surface and a first-stage polishing unit including a first polishing head, while supplying a first polishing liquid to the first polishing pad, the first polishing is performed. A pre-stage polishing step of polishing the surface of the silicon wafer by rotating the first platen and the silicon wafer in a state where the silicon wafer held by the head is in contact with the first polishing pad,
Then, the second polishing pad is supplied with a second polishing liquid including a second polishing plate having a second polishing pad on the surface thereof and a second polishing head, while the second polishing liquid is supplied to the second polishing pad. A final polishing step of further polishing the surface of the silicon wafer by rotating the second surface plate and the silicon wafer in a state where the silicon wafer held by the head is in contact with the second polishing pad;
A method for polishing a silicon wafer, which comprises performing as a final polishing step,
In the pre-polishing step, as the first polishing liquid, first, a first alkaline aqueous solution containing abrasive grains having a density of 1×10 ¹⁴ particles/cm ³ or more is supplied, and thereafter, a water-soluble polymer and a density of Switching to supplying a second alkaline aqueous solution containing 5×10 ¹³ particles/cm ³ or less of abrasive grains,
After the pre-stage polishing step, the silicon wafer is removed from the first polishing head, transported to the final polishing unit while supplying water to the surface of the silicon wafer, and attached to the second polishing head,
In the final polishing step, a third alkaline aqueous solution containing a water-soluble polymer and abrasive grains having a density of 5×10 ¹³ particles/cm ³ or less is supplied as the second polishing liquid. Wafer polishing method.

(2) The silicon according to (1) above, wherein in the preceding polishing step, the supply is switched from the first alkaline aqueous solution to the second alkaline aqueous solution after a polishing time for achieving the target polishing amount in the step has elapsed. Wafer polishing method.

(3) The method for polishing a silicon wafer according to the above (1) or (2), wherein, in the preceding polishing step, the period of supplying the second alkaline aqueous solution is 10 seconds or more.

(4) In the final polishing step, an alkaline stock solution containing the water-soluble polymer and the abrasive grains is mixed with pure water in a dilution tank to prepare the third alkaline aqueous solution,
The prepared third alkaline aqueous solution is supplied to the finish polishing unit by a piping facility communicating with the dilution tank,
Furthermore, before the third alkaline aqueous solution in the dilution tank is exhausted, the alkaline stock solution and the pure water are newly charged into the dilution tank to prepare a new third alkaline aqueous solution. )-The polishing method of the silicon wafer as described in any one of (3).

(5) The silicon wafer according to (4), wherein the alkaline stock solution and the pure water are newly charged into the dilution tank when the third alkaline aqueous solution remains 10% or more of the volume of the dilution tank. Polishing method.

(6) The polishing of the silicon wafer according to any one of (1) to (5) above, wherein the average primary particle diameter of the abrasive grains is in the range of 10 to 70 nm in the first to third alkaline aqueous solutions. Method.

(7) The method for polishing a silicon wafer according to any one of (1) to (6), wherein the abrasive grains contain SiO ₂ particles in the first to third alkaline aqueous solutions.

(8) The first alkaline aqueous solution contains at least one alkali selected from potassium hydroxide, sodium hydroxide, tetramethylammonium hydroxide, and tetraethylammonium hydroxide ,
The method for polishing a silicon wafer according to any one of (1) to (7) above, wherein the second and third aqueous alkali solutions contain ammonia.

(9) In the second and third aqueous alkaline solutions, the water-soluble polymer is one or more selected from hydroxyethyl cellulose, polyethylene glycol, and polypropylene glycol, and any one of (1) to (8) above. A method of polishing a silicon wafer according to claim 1.

According to the method for polishing a silicon wafer of the present invention, it is possible to suppress not only PID but also the generation of scratches having a small step.

It is a flow figure showing a manufacturing process of a silicon wafer including a polishing method of a silicon wafer by one embodiment of the present invention. FIG. 3 is a schematic diagram showing a polishing liquid supply mechanism used in a final polishing step in the method for polishing a silicon wafer according to one embodiment of the present invention. (A), (B), (C), and (D) are LPD maps obtained in Comparative Example 1, Comparative Example 2, Invention Example 1, and Invention Example 2, respectively.

A method of polishing a silicon wafer according to an embodiment of the present invention will be described with reference to FIG. A silicon wafer is manufactured by the flow shown in FIG. In the pre-process of step S1, a slicing process, a lapping process, a chamfering process, an etching process, etc. are performed, and a wafer shape is created by double-side polishing (DSP process) in step S2. The double-side polished silicon wafer is subjected to cleaning in step S3 and then subjected to a final polishing step including a preliminary polishing step in step S4 and a final polishing step in step S5. The silicon wafer that has undergone the final polishing process is subjected to the inspection of the flatness of the wafer and the presence of visible scratches and stains in step S7 after the cleaning in step S6, and then the surface inspection in step S9 after the final cleaning process in step S8. And shipped.

The silicon wafer polishing method of the present embodiment relates to the final polishing step in the above steps. The final polishing process includes a two-step polishing process including a preliminary polishing process S4 performed in the preliminary polishing unit and a final polishing process S5 performed in the final polishing unit thereafter.

In the pre-stage polishing step S4, while supplying the first polishing liquid to the first polishing pad by using a pre-stage polishing unit including a first surface plate having a first polishing pad on the surface and a first polishing head, The surface of the silicon wafer is polished by rotating the first surface plate and the silicon wafer while the silicon wafer held by the first polishing head is in contact with the first polishing pad.

As the first polishing liquid used in the first-stage polishing step, it is preferable to use a polishing liquid that has a relatively high etching rate with alkali and has a large number of abrasive grains. As such a polishing liquid, in the present embodiment, a first alkaline aqueous solution containing abrasive grains having a density of 1×10 ¹⁴ particles/cm ³ or more is used. The polishing rate for silicon in the first alkaline aqueous solution is preferably 100 to 300 nm/min. If it is 100 nm/min or more, the productivity will not be deteriorated, and if it is 300 nm/min or less, the roughness of the wafer surface will not be rough, and the wafer surface can be uniformly polished. .. From the viewpoint of obtaining such a polishing rate, the first alkaline aqueous solution is potassium hydroxide (KOH), it is selected from sodium hydroxide (NaOH), tetramethylammonium hydroxide (TMAH), and tetraethylammonium hydroxide (TEAH) It is preferable to contain at least one kind of alkali, and not to include a water-soluble polymer. The density of the abrasive grains is not particularly limited as long as it is 1×10 ¹⁴ particles/cm ³ or more, but it is preferably 1×10 ¹⁵ particles/cm ³ or less from the viewpoint of suppressing aggregation of the abrasive particles. Here, in the present embodiment, after the first alkaline aqueous solution is supplied and polished, the second alkaline aqueous solution having the same specifications as the third alkaline aqueous solution is supplied as described later.

The wafer immediately after the first-stage polishing step is removed from the first polishing head and is transported to the finish polishing unit while supplying water to the surface of the wafer so that the surface does not dry. Hereinafter, this transportation is also simply referred to as “underwater transportation”. Specifically, the wafer is housed in a transfer container and transferred to the finish polishing unit while water is applied to the surface of the wafer. Alternatively, the wafer transfer passage may be filled with water and transferred therein. Alternatively, the wafer may be stored in a transfer container, the container may be filled with water, and the container may be transferred.

In the final polishing step S5, a second polishing liquid is supplied to the second polishing pad by using a final polishing unit including a second surface plate having a second polishing pad on the surface and a second polishing head. The surface of the silicon wafer is further polished by rotating the second surface plate and the silicon wafer while the silicon wafer held by the second polishing head is in contact with the second polishing pad.

As the second polishing liquid used in the finish polishing step, it is preferable to use a polishing liquid that has a low etching rate with alkali and a small number of abrasive grains. In this embodiment, a third alkaline aqueous solution containing a water-soluble polymer and abrasive grains having a density of 5×10 ¹³ particles/cm ³ or less is used as such a polishing liquid. The polishing rate for silicon in the third alkaline aqueous solution is preferably 5 to 20 nm/min. If it is 5 nm/min or more, the polishing time for obtaining the desired polishing amount does not become long, so that the productivity is not deteriorated, and the defects formed on the wafer surface in the previous polishing step are removed. You can get the full effect. When it is 20 nm/min or less, the etching effect of alkali does not become excessive, and the roughness of the wafer surface does not deteriorate. From the viewpoint of obtaining such a polishing rate, the third alkaline aqueous solution preferably contains ammonia and contains a water-soluble polymer. As the water-soluble polymer, it is preferable to use one or more selected from hydroxyethyl cellulose (HEC), polyethylene glycol (PEG), and polypropylene glycol (PPG). The density of the abrasive grains is not particularly limited as long as it is 5×10 ¹³ particles/cm ³ or less, but from the viewpoint of ensuring the minimum polishing ability and reliably improving the surface roughness of the wafer surface, it is 2×10 ¹³ The number/piece is preferably at least ³ /cm ³ . Since the third alkaline aqueous solution contains a water-soluble polymer, the viscosity is higher than that of the first alkaline aqueous solution, but the viscosity at the operating temperature (18 to 25°C) is 1.5 to 5.0 mPa·s. Preferably. If the viscosity is less than 1.5 mPa·s, the polishing liquid may easily flow, and the desired etching rate may not be obtained. If the viscosity is 5.0 mPa·s or more, polishing may be performed even after cleaning after finish polishing. This is because the liquid may remain and adhere to the wafer surface.

Here, in this embodiment, as shown in FIG. 1, in the pre-stage polishing step, it is important to switch from the supply of the pre-stage polishing slurry (step S4A) to the supply of the finish polishing slurry (step S4B). That is, in the first-stage polishing step, as the first polishing liquid, first, the above first alkaline aqueous solution is supplied, and then the second alkaline aqueous solution having the same specifications as the above third alkaline aqueous solution is supplied. The second alkaline aqueous solution has the same specifications as the third alkaline aqueous solution used in finish polishing, and it is essential that the second alkaline aqueous solution contains a water-soluble polymer and abrasive grains having a density of 5×10 ¹³ particles/cm ³ or less. The other requirements are the same as those of the third alkaline aqueous solution described above.

In the present embodiment, such switching of the polishing liquid supply can suppress not only PID but also the generation of scratches with a low step. The inventors consider the action as follows. ..
(1) Since the second alkaline aqueous solution has less abrasive grains than the first alkaline aqueous solution, aggregation of the abrasive grains remaining on the wafer surface does not easily occur during the underwater transportation from the pre-stage polishing unit to the final polishing unit.
(2) Since the water-soluble polymer contained in the second alkaline aqueous solution protects the wafer surface during the underwater transportation, aggregation of the abrasive grains remaining on the wafer surface during the underwater transportation hardly occurs.
(3) Since the composition of the liquid (second alkaline aqueous solution) remaining on the surface of the wafer and the composition of the second polishing liquid (third alkaline aqueous solution) to be supplied are close to each other when starting the final polishing, Since the pH of the second polishing liquid does not easily fluctuate on the surface, aggregation of abrasive grains does not easily occur.
(4) It is important that the second alkaline aqueous solution contains abrasive grains although it is less than in the first alkaline aqueous solution. Since the concentration of the abrasive grains in the first alkaline aqueous solution used in the preliminary polishing is high, the abrasive grains are likely to aggregate and adhere to the wafer. Here, if the second alkaline aqueous solution does not contain abrasive grains and contains only a water-soluble polymer unlike the protective film-forming liquid described in Patent Document 2, it is highly water-soluble on the wafer surface. A protective film of molecules is formed, but this alone cannot sufficiently remove the abrasive grains attached to the wafer. In the present embodiment, the abrasive particles in the second alkaline aqueous solution are agglomerated on the wafer surface during the pre-step polishing to remove the adhered abrasive particles, so that they are attached to the wafer surface during the subsequent transportation and the stage of the final polishing start. Fewer abrasive grains are present. From this point of view, as described above, it is preferable that the density of the abrasive grains in the second alkaline aqueous solution is 2×10 ¹³ particles/cm ³ or more.

The operations (1) and (2) are obtained on the premise that the pre-stage polishing unit is transported to the finish polishing unit by underwater transport. That is, in the present embodiment, underwater transportation is one of the important steps in order to suppress the occurrence of scratches with a small step. In addition, the water conveyance does not generate a watermark due to the first alkaline aqueous solution used in the front-end polishing being dried and concentrated on the wafer surface. Further, even if the abrasive grains in the first alkaline aqueous solution used in the pre-stage polishing adhere to the side surface of the wafer or the chamfered portion, they can be removed by the underwater transportation.

The timing of switching the supply of the polishing liquid is not particularly limited, but in the pre-stage polishing step, the supply of the first alkaline aqueous solution to the second alkaline aqueous solution is switched after the polishing time for achieving the target polishing amount in the step has elapsed. Is preferred.

In addition, from the viewpoint of sufficiently obtaining the above-described action and effect by switching the supply of the polishing liquid, it is preferable that the second alkaline aqueous solution is supplied for 10 seconds or more in the pre-polishing step. Although the upper limit of the period is not particularly limited, it is preferably 300 seconds or less from the viewpoint of productivity.

In the first to third alkaline aqueous solutions, the average primary particle diameter of the abrasive grains is preferably in the range of 10 to 70 nm. If the particle size is less than 10 nm, the abrasive particles agglomerate into coarse particles having a large particle size, which may cause PID. On the other hand, if the particle size exceeds 70 nm, the particle size is too large. This is because the roughness of the wafer surface after polishing may be deteriorated. In the present specification, the "average primary particle diameter" is determined by the BET method (a method of adsorbing a molecule whose adsorption occupation area is known on the surface of powder particles at the temperature of liquid nitrogen and determining the specific surface area of the sample from the amount). The specific surface area is converted to the diameter of the spherical particles.

In the first to third alkaline aqueous solutions, the abrasive grains should be made of ceramics such as silica and alumina, simple substances or compounds such as diamond and silicon carbide, or high molecular polymers such as polyethylene and polypropylene. However, it is preferable to contain SiO ₂ particles for reasons such as low cost, dispersibility in a polishing liquid, and easy control of the particle size of abrasive grains. In addition, as the type of SiO ₂ particles, for example, any of those prepared by a dry method (combustion method/arc method) and a wet method (sedimentation method/sol-gel method) can be used. The shape of the abrasive grains may be spherical, cocoon-shaped or the like.

The first to third alkaline aqueous solutions preferably do not contain an oxidizing agent (hydrogen peroxide (H ₂ O ₂ ), sodium persulfate, etc.). This is because when an oxidant is contained, not only the wafer surface is oxidized and the etching rate is lowered, but also the surface of the abrasive grains is adhered during the oxidation, which causes PID.

As the second polishing pad, materials such as non-woven fabric, suede, polyurethane foam, polyethylene foam, and porous fluororesin can be used.

Next, in the present embodiment, by devising the method of supplying the third alkaline aqueous solution (second polishing liquid) in the finish polishing step as described below, the occurrence of scratches with a small step can be more sufficiently suppressed. You can This point will be described with reference to FIG. FIG. 2 is a schematic diagram showing a supply mechanism of a polishing liquid used in the final polishing step. In the finish polishing step, the third undiluted alkaline solution is prepared by mixing the undiluted alkaline solution containing the water-soluble polymer and the abrasive grains with pure water in the dilution tank 10 and diluting the undiluted alkaline solution several tens of times. To do. At that time, specifically, pure water is put into the dilution tank 10 through the pure water pipe 16, then a desired amount of the alkaline stock solution is put into the dilution tank 10 through the stock solution pipe 14, and then, It is preferable to add pure water again. If pure water is supplied from above after first putting the alkaline stock solution into the dilution tank 10, foaming occurs due to the effect of the water-soluble polymer in the alkaline stock solution, and the agglomeration of the abrasive grains in the polishing solution easily occurs. Become. Therefore, first, pure water is supplied to make the pure water level higher than the discharge port of the stock solution pipe 14 in the tank, then the alkaline stock solution is supplied, and then the pure water is supplied again. Then, the mixed solution of pure water and the alkaline stock solution is stirred. The method of stirring the mixed solution is not particularly limited, and for example, a circulation pump can be used, and a stirring device equipped with a stirrer of any shape and its driving means may be arranged in the tank. ..

The prepared third alkaline aqueous solution is supplied to the polishing apparatus (finishing polishing unit) by the piping facility communicating with the dilution tank 10. Specifically, the third alkaline aqueous solution is transferred to the finish polishing unit via the supply pipe 18 connected to the lower portion of the dilution tank 10. When the second alkaline aqueous solution used in the pre-polishing step has the same specifications as the third alkaline aqueous solution, the pre-polishing of the third alkaline aqueous solution is performed via the supply pipe 18 connected to the lower portion of the dilution tank 10. It may be transferred to a unit and used as the second alkaline aqueous solution.

Here, all of the prepared third alkaline aqueous solution is transferred from the dilution tank 10 to the finishing polishing unit, and after the dilution tank 10 is emptied, a new alkaline stock solution and pure water are newly introduced into the dilution tank and mixed, It has been found that when a new (fresh) third alkaline aqueous solution is prepared, the pH in the solution is greatly changed, and as a result, the abrasive grains are aggregated, which causes scratches with a small step. Therefore, in the present embodiment, before the third alkaline aqueous solution in the dilution tank 10 is exhausted, new alkaline stock solution and pure water are charged into the dilution tank 10 to prepare a new third alkaline aqueous solution. This makes it possible to reduce fluctuations in pH in the liquid, and as a result, it becomes difficult for the abrasive grains to aggregate, and it is possible to more sufficiently suppress the occurrence of scratches with a low level difference.

From the viewpoint of sufficiently obtaining such effects, it is preferable to newly add the alkaline stock solution and pure water to the dilution tank 10 when the third alkaline aqueous solution remains at 10% or more of the volume of the dilution tank 10. From the viewpoint of productivity, it is preferable to newly add the alkaline stock solution and pure water to the dilution tank 10 when the third alkaline aqueous solution remains at 50% or less of the volume of the dilution tank 10.

The silicon wafer that has been subjected to the final polishing process described above is rinsed and stored in water, and is then subjected to cleaning in step S6 (typically, sulfuric acid+ozone cleaning) within 24 hours after the end of the final polishing process. PID can be suppressed by carrying out water storage and cleaning before the components of the polishing liquid and the abrasive particles are fixed to the surface of the wafer.

In addition, in the above-described embodiment, as shown in FIG. 1, an example is shown in which one pre-stage polishing step S4 performed in one pre-stage polishing unit and two stages of finish polishing step S5 performed in the subsequent finish polishing unit are performed. However, the present invention is not limited to the above embodiment, and the pre-stage polishing step may be performed by two or more pre-stage polishing units. That is, a plurality of pre-stage polishing steps are performed by a plurality of pre-stage polishing units, and the last pre-stage polishing step among them can be equivalent to S4 of FIG.

(Comparative Example 1)
Twenty-five silicon wafers having a diameter of 300 mm, which had been subjected to cleaning after double-side polishing according to a standard method, were subjected to the final polishing step under the following conditions. In the first-stage polishing step, the first polishing liquid contains no water-soluble polymer, contains TMAH as an alkali, and has a density of 2.5×10 ¹⁴ particles/cm ³ (SiO _{2 having an} average primary particle size of 35 nm. The first alkaline aqueous solution containing particles) was supplied. The polishing rate for silicon in this first alkaline aqueous solution is 200 nm/min. The polishing time was 300 seconds.

In the final polishing step, as the second polishing liquid, HEC as a water-soluble polymer, ammonia as an alkali, and an abrasive grain having a density of 5×10 ¹³ particles/cm ³ (SiO ₂ particles having an average primary particle diameter of 35 nm) are used. ) Containing the 3rd alkaline aqueous solution was supplied. The polishing rate for silicon in this third alkaline aqueous solution is 10 nm/min, and the viscosity at 25° C. is 3 mPa·s. The polishing time was 300 seconds. As the second polishing liquid supply method in the finish polishing step, a new polishing liquid was prepared by newly adding an alkaline stock solution and pure water into the dilution tank after the dilution tank became empty and mixing them. ..

The silicon wafer that had undergone the final polishing step was subjected to cleaning, inspection, and final cleaning steps according to a standard method, and PID and scratches with low steps were evaluated by surface inspection as follows.

(Invention Example 1)
A silicon wafer was polished under the same conditions as in Comparative Example 1 except for the following points, and PID and scratches with low steps were evaluated. That is, in Inventive Example 1, after performing the polishing for 300 seconds using the first alkaline aqueous solution in the preliminary polishing step, the supply of the polishing liquid was switched to the second alkaline aqueous solution having the same specifications as the third alkaline aqueous solution. Then, polishing was further performed for 30 seconds.

(Comparative example 2)
The silicon wafer was polished under the same conditions as in Inventive Example 1 except for the following points, and the PID and scratches with low steps were evaluated. That is, in Comparative Example 2, as the second alkaline aqueous solution, an alkaline aqueous solution having the same specifications as the third alkaline aqueous solution except that abrasive grains were not used was used.

(Invention Example 2)
The silicon wafer was polished under the same conditions as in Inventive Example 1 except for the following points, and the PID and scratches with low steps were evaluated. That is, in Example 2 of the present invention, as a method of supplying the second polishing liquid in the finish polishing step, before the third alkaline aqueous solution in the dilution tank is exhausted (specifically, the third alkaline aqueous solution is equal to the volume of the dilution tank). At the timing of remaining 20%), an alkaline stock solution and pure water were newly introduced into the dilution tank and mixed to prepare a new polishing solution.

<Evaluation of PID>
The surface of each wafer was measured with a laser particle counter (SP2, manufactured by KLA Tencor Corp.), and a defect classified as LPD-N with a size of 35 nm or more was identified as PID, and the number thereof was counted. On an average of 25 sheets, Comparative Example 1 was 3, Comparative Example 2 was 3, Invention Example 1 was 2, and Invention Example 2 was 1. As described above, in all of Comparative Examples 1 and 2 and Invention Examples 1 and 2, generation of PID could be sufficiently suppressed.

<Evaluation of scratches with low steps>
The surface of each wafer is measured by the normal mode of a laser particle counter (KLA Tencor, SP3), and a LPD detected as a defect having a size of 36 nm or more is used to create an in-wafer map. At that time, As shown in FIG. 3, defects detected were continuous and observed in a long linear shape was evaluated as a thin scratch. Note that FIGS. 3A to 3D are obtained by superimposing LPD maps of 25 wafers. In Comparative Example 1, scratches occurred in 15 out of 25 wafers, and as is clear from FIG. 3A, it was long and had a length of about 140 mm, which was close to the radius of the wafer. In Comparative Example 2 as well, scratches occurred in 10 out of 25 sheets. On the other hand, in Inventive Example 1, scratches occurred in 10 out of 25 sheets, the scratches were considerably reduced as compared with Comparative Examples 1 and 2, and the detected lengths of the scratches were as in Comparative Examples 1 and 2. It was very short in comparison. In Invention Example 2, none of the 25 sheets had scratches.

According to the method for polishing a silicon wafer of the present invention, it is possible to suppress not only PID but also the generation of scratches having a small step.

10 Dilution tank 14 Pipe for undiluted solution 16 Pipe for pure water 18 Pipe for supply

Claims (9)

Perform a double-sided polishing process as a rough polishing process on the silicon wafer, and then
Using a pre-stage polishing unit including a first surface plate having a first polishing pad on the surface thereof and a first polishing head, the first polishing head is operated by the first polishing head while supplying the first polishing liquid to the first polishing pad. by rotating the first plate and the silicon wafer to the silicon wafer was kept being in contact with the first polishing pad, the front polishing step of polishing the surface of the silicon wafer,
After that, the second polishing pad is supplied with a second polishing liquid by using a finish polishing unit including a second surface plate having a second polishing pad on its surface and a second polishing head. A final polishing step of further polishing the surface of the silicon wafer by rotating the second surface plate and the silicon wafer in a state where the silicon wafer held by the head is in contact with the second polishing pad;
A method for polishing a silicon wafer, which comprises performing as a final polishing step,
In the preceding polishing step, as the first Institute Migakueki, first, the density is supplied to the first alkaline aqueous solution containing 1 × 10 ¹⁴ atoms / cm ³ or more of the abrasive grains, the polishing rate is 100 to 300 nm / min Polishing is performed , and then the supply of a second alkaline aqueous solution containing a water-soluble polymer and abrasive grains having a density of 5×10 ¹³ particles/cm ³ or less is switched to a polishing rate of 5 to 20 nm/min. Polishing ,
After the pre-stage polishing step, the silicon wafers removed from the first polishing head, and transported to the final polishing unit while supplying water to the surface of the silicon wafer, attached to the second Migaku Ken head,
In the final polishing step, as the second Research Migakueki, and a water-soluble polymer, density and provide a third alkaline aqueous solution containing 5 × 10 ¹³ atoms / cm ³ or less of the abrasive grains, the polishing rate is 5 A method for polishing a silicon wafer, which comprises performing polishing at -20 nm/min . The method for polishing a silicon wafer according to claim 1, wherein in the first-stage polishing step, the supply is switched from the first alkaline aqueous solution to the second alkaline aqueous solution after a polishing time for achieving the target polishing amount in the step has elapsed. .. The method for polishing a silicon wafer according to claim 1, wherein, in the pre-polishing step, a period of supplying the second alkaline aqueous solution is 10 seconds or more. In the final polishing step, an alkaline stock solution containing the water-soluble polymer and the abrasive grains is mixed with pure water in a dilution tank to prepare the third alkaline aqueous solution,
The prepared third alkaline aqueous solution is supplied to the finish polishing unit by a piping facility communicating with the dilution tank,
Furthermore, before the third alkaline aqueous solution in the dilution tank is exhausted, the alkaline stock solution and the pure water are newly charged into the dilution tank to prepare a new third alkaline aqueous solution. 4. The method for polishing a silicon wafer according to any one of 3 to 3. The method for polishing a silicon wafer according to claim 4, wherein the alkaline stock solution and the pure water are newly charged into the dilution tank when 10% or more of the volume of the dilution tank remains in the third alkaline aqueous solution. The method for polishing a silicon wafer according to claim 1, wherein an average primary particle diameter of the abrasive grains in the first to third alkaline aqueous solutions is in a range of 10 to 70 nm. The method for polishing a silicon wafer according to claim 1, wherein the abrasive grains contain SiO ₂ particles in the first to third alkaline aqueous solutions. The first alkaline aqueous solution contains at least one alkali selected from potassium hydroxide, sodium hydroxide, tetramethylammonium hydroxide, and tetraethylammonium hydroxide ,
The method for polishing a silicon wafer according to claim 1, wherein the second and third aqueous alkali solutions contain ammonia. The silicon according to any one of claims 1 to 8, wherein the water-soluble polymer in the second and third alkaline aqueous solutions is one or more selected from hydroxyethyl cellulose, polyethylene glycol, and polypropylene glycol. Wafer polishing method.

Images